Remove superfluous casts

-  Every non-const pointer converts automatically to void *.
-  Every pointer converts automatically to void *.
-  void * converts to any other pointer.
-  const void * converts to any other const pointer.
-  Integer variables convert to each other.

I changed the declaration of a few variables in order to allow removing
a cast.

However, I didn't attempt to edit casts inside comparisons, since they
are very delicate.  I also kept casts in variadic functions, since they
are necessary, and in allocation functions, because I have other plans
for them.

I also changed a few casts to int that are better as ptrdiff_t.

This change has triggered some warnings about const correctness issues,
which have also been fixed in this patch (see for example src/login.c).

Signed-off-by: Alejandro Colomar <alx@kernel.org>

libmisc/chowntty.c

@@ -51,7 +51,7 @@ void chown_tty (const struct passwd *info)
 	 */
 
 	if (   (fchown (STDIN_FILENO, info->pw_uid, gid) != 0)
-	    || (fchmod (STDIN_FILENO, (mode_t)getdef_num ("TTYPERM", 0600)) != 0)) {
+	    || (fchmod (STDIN_FILENO, getdef_num ("TTYPERM", 0600)) != 0)) {
 		int err = errno;
 		FILE *shadow_logfd = log_get_logfd();